The present invention relates to cable modems. More specifically, the present invention relates to voice transmission over cable modems. The Data Over Cable Systems Interface Specification (DOCSIS) describes a standard for the cable modem interface between a cable TV operator and a computer. DOCSIS has been accepted as the standard for devices that handle incoming and outgoing data signals across this interface since DOCSIS 1.0 was ratified by the International Telecommunication Union (ITU) in March of 1998. Cable modems conforming to DOCSIS are available in many areas where cable operators operate.
The DOCSIS 1.1 specification provides for Unsolicited Grant service as one of several data transmission mechanisms for grabbing bandwidth for packets. This service is set up ahead of time between a cable modem and head-end equipment (CMTS), and is parameterized by the desired data packet arrival rate, packet size, and jitter tolerance. Once set up, the cable network head-end equipment will give unsolicited transmission opportunities (grants) to the cable modem at the requested rate. This service provides an efficient mechanism for transporting voice packets but does not provide for maximizing voice quality.
Unlike callers in a circuit-switched telephone network who are allocated a dedicated connection, cable modem users do not occupy a fixed amount of bandwidth during their online sessions but instead share the network with other online users, only using network resources when sending and receiving data in quick bursts. Users typically grab what they need from the available bandwidth at the instant they need to upload their packets. Delays in transmitting data packets do not affect data quality. However, for voice packets, delays in transmission degrade voice quality so there is a need to closely time voice packet availability with opportunities for transmission, especially in cable networks.
DOCSIS is an evolving standard which specifies modulation schemes and protocols for exchange of bidirectional signals over cable, allowing version 4 IP traffic to achieve transparent transfer between the Cable Modem Termination System-Network Side Interface (CMTS-NSI) and the Cable Modem to Customer Premise Equipment Interface (CMCI). Voice over IP (VoIP) is a new service capable of being supported by DOCSIS cable networks. Upgrades to existing cable modems and DSPs to maximize VoIP quality can be achieved by changing the programming in their EEPROM flash memory.